Method of using an optic relay comprising an element of an electro-optical material and a photosensitive element

ABSTRACT

In order to erase an image recorded on an optic relay comprising an element of electro-optical material and a photosensitive element, the photosensitive layer is uniformly exposed and simultaneously is applied to the relay a voltage of a sign opposite to that of the voltage previously applied during the recording and having such a value that during the re-recording the voltage at the terminals of the electro-optical element is zero. Application: Television.

Grenot et al.

[4 1 Dec. 17, 1974 METHOD OF USING AN OPTIC RELAY COMPRISING AN ELEMENTOF AN ELECTRO-OPTICAL MATERIAL AND A PHOTOSENSITIVE ELEMENT Inventors:Michel Grenot, BrunOy; Jean Pergrale, Paris, both of France Assignee:U.S. Philips Corporation, New

York, NY.

Filed: Jan. 15, 1973 Appl. No.: 323,848

Foreign Application Priority Data Jan. 14. I972 France 72401326 U.S.Cl.. 340/173 LM, 340/173 LS, 350/160 R [56] References Cited UNITEDSTATES PATENTS 3,740,734 6/1973 Maldonado 340/173 LM PrimaryExaminer-Terrell W. Fears Attorney, Agent, or Firm-Frank R. Trifari;Simon L. Cohen [57] ABSTRACT In order to erase an image recorded on anoptic relay comprising an element of electro-optical material and aphotosensitive element, the photosensitive layer is uniformly exposedand simultaneously is applied to the relay a voltage of a sign oppositeto that of the voltage previously applied during the recording andhaving such a value that during the re-recording the voltage at theterminals of the electro-optical element Int. Cl. Gllc 13/04 is ZeroApplication: Television Field of Search 340/173 LM, l73 L5 2 Claims, 1Drawing Figure METHOD OF USING AN OPTIC RELAY COMPRISING AN ELEMENT OFAN ELECTRO-OPTICAL MATERIAL AND A PHOTOSENSITIVE ELEMENT The presentinvention is related to the invention which forms the object of the US.Pat. 3,792,259 in the name of applicant. It relates more in particularto a method of using an optic relay described in the above patentapplication when in the manufacture of said relay a photosensitiveelement is included.

In the said patent application is stated in particular a method oferasing an image recorded in the relay, which erasing is necessary whenone image is to be re placed by another.

For erasing a recorded image, the method described consists in exposingthe photosensitive layer in a uniform manner, for example, by means of aflash of light, and simultaneously removing the direct voltage appliedbetween the two control electrodes. In the case in which thephotosensitive layer is a photoconductor, said control electrodes areconstituted by two transparent conductive layers deposited on the facesof the stack consisting of the plate of electro-optical material, thedielectric mirror and the photoconductive layer. Under the influence ofthe light, numerous pairs of electrons-holes are produced in thephotoconductor the resistivity of which decreses considerable.

The phenomenon or erasing charges deposited on the electro-opticalmaterial may be made equal to a capacitor discharge via a resistor whichmay be non-linear, the capacitor being constituted by the plate ofelectrooptical material and the resistance being that of thephotoconductive element. The erased" state is obtained while the appliedexternal voltage is zero. When with a view to a new erasing said voltageis brought to the recording value, the capacitive divider comprising inseries on the one hand the capacity C constituted by the assembly plateof electro-optical material dielectric mirror and on the other hand thecapacity C constituted by the photoconductive element causes a volt ageequal to V C'/C C to appear at the terminals of the said plate, whichvoltage is sufficient to produce a uniform modulation to which a uniformbackground corresponds. So the contrast of any new image is influencedby the existence of said uniform background. This phenomenon appearsboth in the case in which the photosensitive layer is a photocathode andin the case in which it is a photoconductor. Hence, it is generally moreimportant in said latter case.

The method of using the optic relay according to the present inventionendeavours to mitigate said inconvenience. It is remarkable in that, inorder to erase a recorded image, the photosensitive layer is uniformlyexposed and simultaneously a voltage -V having a sign opposite to thatof the voltage V applied during the recording is applied between thecontrol electrodes. The value of V is such that when the voltage V isagain applied for a new recording, the voltage which appears at theterminals of the useful capacity C of the assembly electro-opticalmaterial-dielectric mirror is zero. Said value of V is equal to C'/C V.

The invention will be better understood with the aid of the followingdescription of the development of a few particular phases of the methodof using the optic relay the constitution of which is repeated, formemory, on the sole FIGURE accompanying the said description in the casein which the photosensitive layer is a photoconductor.

It will be obvious that any conception of the method of using the relaycomprising the same stages of changing the state of change of the plateof electro-optical material obtained with the same means but in adifferent sequence form part of the invention.

In the sole FIGURE, reference numeral 1 denotes a sheet of materialwhich is transparent to the reading light and which serves to supportthe assembly of the relay, 2 is a conductive layer which is transparentto the reading light and permits the connection of the contact 3deposited on the plate 4 of electro-optical material, for example,deuterated potassium biph osphate (KDP), 5 is the multi-layer dielectricmirror permitting the reflection of the reading light emitted by thesource 6.

The photoconductive layer is shown at 7, the conductive layer at 8 whichis transparent to the erasing light 9 and permits the connection to thecontact 10.

During the recording of an image, a direct voltage +V is applied betweenthe electrodes 3 and 10.

By means of a cooling device not shown in the FIG- URE, the KDP isbrought at a temperature near its 'Curie point (approximately 50C).

In a first stage of the step of using the relay, the assumed initialcharging state of the plate corresponds to a non-uniform charge densitywhich may originate, for example, from a first image erasing byprojection with the aid of the light 9 in the presence of the appliedpotential +V.

In a second stage, in order to erase the image the voltage V (C/C V) isapplied between the electrodes 3 and 10 and the photosensitive layer isexposed with the aid ofa beam of uniform light 9 during a time which issufficient for the assembly KDP-dielectric mirror to be charged to thevoltage -V'.

In a third stage, the recording conditions of an image are whileapplying the voltage +V between the electrodes 3 and I0 and projectingan image on the photosensitive layer. Due to the capacity existingbetween the faces of the photosensitive layer, the various points of theassembly KDP-dielectric mirror are brought at potentials which varybetween 0 for the non-exposed points and +V for the most exposed points.

What is claimed is:

1. A method of using an optic relay in conjunction with at least a firstsource of light radiation having a first frequency, means for projectingthe said radiation, at least a second source of light radiation having asecond frequency, polarization and projection means from said secondlight radiation on an optic image relay constituted by an evacuatedspace having at least one window which is transparent to the said lightradiation, a layer which is photosensitive to the said first radiation,a plate having an electro-optic effect maintained at a temperature nearits Curie point and presenting a variable double refraction as afunction of a potential difference existing between its faces, a firstelectrode which is electrically conductive and optically transparent tothe said second light radiation being disposed against the said plateand a second electrode being placed on the opposite side in theproximity of the said plate, the method comprising displaying an imageof electric charges while projecting an image on the photosensitivelayer and in the presence of a voltage V between the electrodes, erasingsaid charges image while isting between said assembly and the electrodesituated on the side opposite to the plate, in such manner that thevoltage at the terminals of the assembly electrooptical plate-dielectricmirror is removed when the voltage V is again applied between theelectrodes in the absence of light on the photosensitive layer.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONDecember l7,

PATENT NO.

DATED 1974 INVENTOWS) MICI-IEL GRENOT ET AL it is certified that errorappears in the ab0ve-identified patent and that said Letters Patent arehereby corrected as shown below:

IN THE SPECIFICATION ine 42, "V C'/C+C should read V line 62, "C'/C V"should read L Col. 2, line 33, "V' (C'/C V)" should read V' V),'

IN THE CLAIMS Claim 2, line 2, "V' V C'/C" should read V' V Signed andScaled this twenty-third Day Of September1975 [SEAL] Arzesr:

C. MARSHALL DANN ('mnmisxinncr of Iulcnlx and Trademarks RUTH C. MASONAlresting Officer

1. A method of using an optic relay in conjunction with at least a firstsource of light radiation having a first frequency, means for projectingthe said radiation, at least a second source of light radiation having asecond frequency, polarization and projection means from said secondlight radiation on an optic image relay constituted by an evacuatedspace having at least one window which is transparent to the said lightradiation, a layer which is photosensitive to the said first radiation,a plate having an electro-optic effect maintained at a temperature nearits Curie point and presenting a variable double refraction as afunction of a potential difference existing between its faces, a firstelectrode which is electrically conductive and optically transparent tothe said second light radiation being disposed against the said plateand a second electrode being placed on the opposite side in theproximity of the said plate, the method comprising displaying an imageof electric charges while projecting an image on the photosensitivelayer and in the presence of a voltage V between the electrodes, erasingsaid charges image while illuminating uniformly the photosensitivelayer, and simultaneously applying between the two electrodes a voltage-V''.
 2. A method as claimed in claIm 1, characterized in that theapplied voltage -V'' has a value -V'' V C''/C, in which C is thecapacity of the assembly electro-optical plate-dielectric mirror and C''is the capacity existing between said assembly and the electrodesituated on the side opposite to the plate, in such manner that thevoltage at the terminals of the assembly electro-opticalplate-dielectric mirror is removed when the voltage V is again appliedbetween the electrodes in the absence of light on the photosensitivelayer.